Comprehensive Notes on Haloalkanes and Haloarenes

Properties and Reactivity of Haloalkanes

• Reactivity Order of Alkyl Halides ($R-X$):

  • By halogen type: R-I > R-Br > R-Cl > R-F.

  • By degree of substitution for $S_N1$ reactions: 3^{\circ} > 2^{\circ} > 1^{\circ}.

  • By degree of substitution for $S_N2$ reactions: 1^{\circ} > 2^{\circ} > 3^{\circ}.

• Physical Properties of Haloalkanes:

  • Dipole Moment: The trend is slightly non-linear due to the small size of fluorine. The order is: CH_3-Cl > CH_3-F > CH_3-Br > CH_3-I.

  • Bond Enthalpies: Increases with decreasing atomic size of the halogen: CH_3-F > CH_3-Cl > CH_3-Br > CH_3-I.

  • Boiling Point: Increases with the size and mass of the halogen atom: R-I > R-Br > R-Cl > R-F.

  • Density: Increases as the halogen becomes heavier: n-C_3H_7Cl < n-C_3H_7Br < n-C_3H_7I.

  • Solubility: Haloalkanes are only slightly soluble in water ($H_2O$) despite their polarity, because they cannot form hydrogen bonds with water molecules.

Preparative Methods for Haloalkanes

• From Alcohols ($R-OH$):

  • Alcohols react with reagents like $PCl_5$, $PCl_3$, or $SOCl_2$ (Darzen’s procedure) to yield $R-Cl$.

  • Reaction with $HX$: $R-OH + HX \rightarrow R-X + H_2O$.

• From Alkenes:

  • Free Radical Halogenation: $CH_3-CH=CH_2$ treated with $Cl_2$ at $800\,K$ or $NBS$ (N-Bromosuccinimide) in $CCl_4$ results in allylic substitution: $Cl-CH_2-CH=CH_2$.

  • Addition of $HX$: $CH_3-CH=CH_2 + HBr$ yields $CH_3-CH(Br)-CH_3$ (Markovnikov addition).

  • Peroxide Effect (Kharasch Effect): $CH_3-CH=CH_2 + HBr$ in the presence of peroxide yields $CH_3-CH_2-CH_2-Br$ (Anti-Markovnikov addition).

  • Vicinal Dihalide Formation: $CH_2=CH_2 + Br_2/CCl_4 \rightarrow Br-CH_2-CH_2-Br$.

• Halogen Exchange Reactions:

  • Finkelstein Reaction: $R-X + NaI \xrightarrow{\text{Acetone}} R-I + NaX$ (where $X = Cl, Br$).

  • Swarts Reaction: $R-X$ (usually $R-Cl$ or $R-Br$) reacted with metallic fluorides like $AgF$, $Hg_2F_2$, $CoF_2$, or $SbF_3$ yields $R-F$.

Nucleophilic Substitution Reactions ($S_N$) of Alkyl Halides

Alkyl halides react with various nucleophiles ($Nu^-$) to replace the halogen atom:

• Formation of Alcohols: $R-X + AgOH \text{ (moist } Ag_2O) \rightarrow R-OH$.

• Formation of Ethers (Williamson Synthesis):

  • $R-X + R'ONa \rightarrow R-O-R' + NaX$.

  • $R-X + Ag_2O \text{ (dry)} \rightarrow R-O-R$.

• Formation of Nitriles and Isonitriles (Ambident Nucleophiles):

  • $R-X + KCN \rightarrow R-CN$ (Alkyl cyanide/nitrile).

  • $R-X + AgCN \rightarrow R-NC$ (Alkyl isocyanide).

• Formation of Nitro and Nitrite Compounds:

  • $R-X + KNO_2 \rightarrow R-O-N=O$ (Alkyl nitrite).

  • $R-X + AgNO_2 \rightarrow R-NO_2$ (Nitroalkane).

• Formation of Amines: $R-X + NH_3 \rightarrow R-NH_2$ (Ammonolysis).

• Formation of Thioethers: $R-X + NaSH \rightarrow R-SH$; $R-X + R'SNa \rightarrow R-S-R'$.

• Formation of Esters: $R-X + R'COOAg \rightarrow R'COOR$.

Reduction and Organometallic Reactions

• Reduction to Alkanes:

  • $R-X + LiAlH_4 \rightarrow R-H$ (effective for $1^{\circ}$ and $2^{\circ}$ halides).

  • $R-X + NaBH_4/EtOH \rightarrow R-H$ (effective for $2^{\circ}$ and $3^{\circ}$ halides).

  • $R-X \xrightarrow{HI/P} R-H$.

• Wurtz Reaction: $2R-X + 2Na \xrightarrow{\text{dry ether}} R-R + 2NaX$ (Symmetrical alkanes).

• Wurtz-Fittig Reaction: $Ar-X + R-X + 2Na \xrightarrow{\text{dry ether}} Ar-R$.

• Corey-House Synthesis: Uses lithium dialkyl copper ($(R')_2CuLi$) to react with $R-X$ to form unsymmetrical alkanes ($R-R'$).

• Grignard Reagent Formation: $R-X + Mg \xrightarrow{\text{dry ether}} R-MgX$.

Trihaloalkanes: Chloroform ($CHCl_3$)

• Preparation (Haloform Reaction):

  • Compounds containing the methyl ketone group ($CH_3-C=O$) or alcohols oxidizable to them (like ethanol or Isopropanol) react with $X_2 + NaOH$ (or $NaOX$).

  • Example with Acetone: $CH_3-CO-CH_3 \xrightarrow{Cl_2/OH^-} CCl_3-CO-CH_3 \xrightarrow{OH^-} CHCl_3 + CH_3COO^-$.

• Reactions of Chloroform ($CHCl_3$):

  • Oxidation: In the presence of light and air, it forms Phosgene ($COCl_2$), a poisonous gas. $CHCl_3 + \frac{1}{2}O_2 \xrightarrow{h\nu} COCl_2 + HCl$.

  • Reaction with Silver Powder: $2CHCl_3 + 6Ag \rightarrow HC \equiv CH + 6AgCl$ (Acetylene formation).

  • Nitration: $CHCl_3 + HNO_3 \rightarrow CCl_3NO_2$ (Chloropicrin or Tear gas).

  • Reimer-Tiemann Reaction: Reaction with phenol and $KOH$ to produce Salicylaldehyde.

  • Carbylamine Reaction (Isocyanide Test): Primary amines react with $CHCl_3$ and alcoholic $KOH$ to form foul-smelling isocyanides ($R-NC$).

Haloarenes: Properties and Electrophilic Substitution (ESR)

• Reactivity: Haloarenes are less reactive towards nucleophilic substitution than haloalkanes due to resonance, $sp^2$ hybridization of the carbon, and the instability of the phenyl cation.

• Electrophilic Substitution Reactions (ESR): The halogen atom is deactivating but ortho/para directing.

  • Halogenation: Chlorobenzene + $Cl_2/FeCl_3 \rightarrow$ 1,2-dichlorobenzene (ortho) + 1,4-dichlorobenzene (para).

  • Nitration: Chlorobenzene + $conc. HNO_3/H_2SO_4 \rightarrow$ ortho and para nitrochlorobenzene.

  • Sulfonation: Chlorobenzene + $conc. H_2SO_4 \rightarrow$ chlorobenzene sulfonic acid (ortho/para).

  • Friedel-Crafts Alkylation: Chlorobenzene + $CH_3Cl/AlCl_3 \rightarrow$ 1-chloro-2-methylbenzene + 1-chloro-4-methylbenzene.

  • Friedel-Crafts Acylation: Chlorobenzene + $CH_3COCl/AlCl_3 \rightarrow$ ortho and para chloroacetophenone.

• Nucleophilic Substitution (Benzyne Mechanism):

  • Under drastic conditions (high temperature/pressure), aryl halides can undergo substitution via a benzyne intermediate. Reagent: $NaNH_2$ in liquid $NH_3$.

• DDT Synthesis:

  • Chloral ($CCl_3-CHO$) reacts with two molecules of chlorobenzene in the presence of $conc. H_2SO_4$ to produce $p,p'$-Dichlorodiphenyltrichloroethane (DDT).

• Physical Point on Melting/Boiling Points:

  • Boiling points of isomeric dihalobenzenes are very similar.

  • Melting point of the para-isomer is significantly higher than ortho and meta isomers due to the high symmetry of the para-structure, which allows for better crystal lattice packing.